1. Field of the Invention
The present invention relates to a lithographic apparatus and cleaning method therefore. More particularly it relates to a radical cleaning arrangement for a lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of one, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In current designs for lithographic apparatus operating in the EUV range (with wavelengths typically of 5-20 nm) of the electromagnetic spectrum it is desirable to provide highly reflective elements in order to condition and pattern a beam for transferring a pattern from a patterning device onto a substrate and for maintaining a high productivity of the lithographic tool. This part of the electromagnetic spectrum is very sensitive for transmission losses since the radiation is easily absorbed by most surfaces. For high reflectivity Mo/SI multilayer mirrors are made typically including a metal top layer such as a ruthenium (Ru) layer. The metal top layer is to prevent oxidation, but causes a decrease in reflectivity. Furthermore, the sources that are used to produce such EUV radiation are typically plasma sources, wherein current designs in particular use a tin source. These plasma sources have a tendency to produce, in addition to EUV radiation, a variety of debris particles, which may, without appropriate measures, migrate into the system and cause contamination and malfunction.
In particular, tin contamination from the plasma source will deposit on mirror elements and cause serious loss of reflectivity of the mirror. This is prevented by contraptions to catch the debris before it can do any harm. However, these contraptions can have a limited protection efficiency. Typically, such a contraption is a so-called foil trap which is designed to trap debris particles in substantially aligned foil plates. Downstream, relative to the direction of EUV radiation, a collector is typically configured to collect an optimal portion of radiation coming out of the EUV-source. Such a collector is typically an arrangement of mirror elements, which use a grazing incidence reflection for collecting and guiding incident radiation into a beam of EUV radiation. In particular for the mirror elements of the collector, but also for other mirror elements which are subject to contamination, it is therefore desirable to provide cleaning techniques to clean these mirror elements from contamination. One such technique is hydrogen cleaning. In this method, in particular in combination with a tin source, hydrogen radicals react with tin to form gaseous tin-hydrides (SnH4). Another technique is halogen cleaning. Also a combined technique may be used, in particular, wherein tin oxides are reduced to tin using hydrogen, and where the thus formed tin is removed using halogen cleaning.